The present invention is related to a method of removing a liquid from a rotating substrate. This liquid can be any wet processing liquid as e.g. a wet etching liquid or a cleaning liquid. It can also be a rinsing liquid. The invention is applicable for a number of wet processing steps which are frequently used in the fabrication process of integrated circuits or liquid crystal displays.
The complete and efficient removal of a liquid from a surface of a substrate is a multiply repeated step in e.g. the fabrication process of integrated circuits. Such a step can be performed after a wet etching step or a wet cleaning step or a wet rinsing step or any other step used in said fabrication process wherein said substrate is treated or exposed or immersed in a liquid. Said substrate can be a semiconductor wafer or a part thereof or a glass slice or any other slice of an insulating or conductive material.
The manufacturing of integrated circuits evolves towards processing of each substrate individually rather than in batches of several substrates. In state of the art IC manufacturing, most processing steps as e.g. implantation steps, deposition steps are already performed in a single substrate mode. On the other hand, wet processing steps such as e.g. cleaning steps and subsequent liquid removal steps are typically performed in a batch mode because of lack of appropriate alternatives. Therefore, differences in waiting times are created for each individual substrate between a wet processing step, performed in a batch mode and another processing step, performed in a single substrate mode. Such variability is undesirable with regard to process control. Moreover this mixed batch and single substrate processing increases the cycle time, which is also undesirable. Therefore, there is a general interest in the development of competitive single substrate wet processing steps. Particularly, one of the major challenges regarding single wafer wet processing is a method for removing a liquid from both sides of a substrate. There are two major requirements to be fulfilled for such a method. At first the method should work sufficiently fast. Knowing that in state of the art production lines a substrate is processed typically every 2 to 3 minutes, ideally, in order to avoid equipment duplication, the process step and the liquid removal step should be completed in about such a time frame. Another requirement is related to the preferred substrate orientation. State of the art processing equipment and transportation tools are developed to handle substrates in a horizontal position. Therefore in order to avoid additional substrate handling it would be desirable to perform the wet processing steps using horizontally positioned substrates.
In the U.S. Pat. No. 5,556,479, a method is disclosed of drying substrates after treatment in a liquid. After such a treatment the wafers are immersed in rinse water. The wafer are dried by pulling said substrate being immersed in rinse water slowly out of said rinse water while heating the water interface. However, this known method requires that the substrates are pulled out of the rinse water in an upright position, i.e. a surface of said substrate is about perpendicular to the surface of the rinse water bath as can be seen in FIG. 3 of U.S. Pat. No. 5,556,479. This handling is incompatible with the majority of the other process steps where the equipment and transportation tools are developed to handle horizontally positioned substrates.
In the U.S. Pat. No. 5,271,774 a spin-drying technique is disclosed which is able to handle horizontal positioned substrates. In fact several small liquid islands are formed being removed from the substrate by a rotary movement. It is known that such a spin-drying technique leaves undesirable residues, often referred to as drying marks, on the substrate surface, particularly on surfaces having mixed hydrophilic and hydrophobic regions.
In an aspect of the invention a method is disclosed of removing a liquid from at least one surface of at least one substrate comprising the steps of:
subjecting said substrate to a rotary movement
supplying a liquid on at least a part of said surface of said substrate; and
locally heating said liquid on said part of said surface, while supplying said liquid, to thereby locally reduce the surface tension of said liquid. Particularly, by supplying said liquid and by locally heating said liquid on said part of said surface of said substrate, at least locally a sharply defined boundary is created between the liquid and the ambient, i.e. a so-called liquid-ambient boundary. Alternatively, the substrate can be heated locally at the liquid-ambient boundary. The heat is transferred to the liquid on the surface of the substrate near the liquid-ambient boundary to thereby reduce the surface tension of the liquid at this liquid-ambient boundary.
In an embodiment of the invention, said rotary movement is performed at a speed to guide said liquid-ambient boundary over said substrate. Particularly, this speed can be between 2 and 40 revolutions per second or between 1 and 50 revolutions per second or more than 40 revolutions per second. Preferably this boundary is a curved boundary. The configuration is such that the liquid is kept at the outerside of the curved boundary, i.e. at the liquid side of the liquid-ambient boundary. No liquid is present at the ambient side of the liquid-ambient boundary. In an embodiment of the invention the substrate can rotate around its own axis. Alternatively said substrate can also be subjected to a rotary movement where said substrate no longer rotates around its own centre but around and axis parallel to and offset to the axis perpendicular to and through the centre of the substrate.
In another embodiment of the invention, on said surface of said substrate fresh liquid is sprayed continuously. The entire surface at the liquid side of the liquid-ambient boundary can be covered, as e.g. for hydrophilic surfaces, with a continuous film of the liquid. The speed of the rotary movement is chosen such that the flow of said sprayed liquid on at least one side of the wafer is transported outwards due to the centrifugal forces. Moreover, by locally heating said liquid, the resulting surface tension reduction of said liquid facilitates the movement of said liquid towards an edge of the substrate. The surface left behind is cleaned and dried. It is presumed that this drying action is obtained according to at least the combination of the rotary movement and the Marangoni effect. According to the Marangoni effect, by locally heating the liquid a temperature gradient will be created in the liquid meniscus. This temperature gradient creates an additional force exerted on the liquid film in the direction of the liquid film resulting in a good drying performance.
The liquid is selected dependent upon the applied wet processing step like e.g. a wet etching step or a cleaning step or a rinsing step. To initiate the drying process, besides spraying liquid on at least a part of a surface of said substrate, the liquid is locally heated by a heat source to reduce the surface tension of the liquid. Particularly, the heat source can be a nozzle, movable or not, or a static inlet dispensing a heated gas or a heated vapor or a heated mixture of a vapor and a gas. Also other heat sources can be used such as laser beams or other energetic beams, provided that they can be sufficiently localized. A vapor is defined as the gas phase occurrence of an element or of a compound or of a mixture of elements if the element or compound or mixture should be in the liquid or solid phase at the given temperature and pressure conditions. Thus a vapor can co-exist in one environment with the solid or liquid phase of the element. A vapor is a specific gas phase occurrence of an element or a compound or a mixture of elements.
In another embodiment of the invention, another force can be combined with the liquid removal process of the present invention. Particularly, by using sonic energy as said other force to agitate the liquid applied during the removal process the cleaning performance of said liquid removal process can be enhanced. Doing so can help in particle reduction. Alternatively also contacting a surface with a rotating cleaning pad is an example of such other force.
In another embodiment of the invention, eventually prior to the liquid removal step an etching, a cleaning or a rinsing liquid or a sequence of such liquids can be supplied to the entire surface of a rotating substrate. The parameters can be optimized such that a liquid film can completely cover a surface. The spinning motion will quickly transport the liquid over the surface towards the edge, thus allowing relatively short carry-over transients and thus also allowing for relatively short rinsing times. Using such a continuously switched flow of liquids eliminates the undesirable passage of liquid-gas interfaces over the surface. The liquid removal method of the present invention is applicable for each sequence of at least one wet processing step by the liquid in order to reduce the surface tension of the liquid. The drying can thus be applied directly on the processing liquid if relevant for the application. Since the proposed drying technique is found to be very fast, process non-uniformity over the surface can be kept very low.
In an aspect of the invention an apparatus is disclosed for removing a liquid from at least one surface of at least one substrate, said apparatus comprising:
a substrate holder which is subjectable to a rotary movement, said substrate being releasable held by said substrate holder;
at least one liquid supply system for applying a liquid on at least a part of said surface of said substrate;
at least one heat source for locally heating said liquid. Preferably, said heat source and said liquid supply system are positioned such that the position where the heating takes place is closer to the centre of said rotary movement of said substrate holder than the position where said liquid is applied.
In an embodiment of the invention, said apparatus further comprises a chamber, wherein said substrate holder is positioned. This chamber is designed in a manner to avoid back splashing of the liquid removed from a surface onto said surface. For instance, a chamber having slanted walls may be used.
In another embodiment of the invention, said apparatus further comprises a generator of mechanical vibrations and a transmitter for transmitting said vibrational energy to a surface of the substrate via the liquid being supplied at said surface.
In another embodiment of the invention, the heat source is at least one nozzle which dispenses a heated gas or a heated vapor or a heated mixture of a vapor and a gas on said surface of said substrate and said liquid supply system can comprise at least one-nozzle for applying said liquid on said surface of said substrate, said nozzles are positioned such that the position where the heating is performed is closer to the centre of the rotary movement of the substrate holder than the position where the liquid is applied. Particularly, at least locally a sharply defined liquid-ambient boundary can be created which is located inbetween a first and a second adjacent nozzle, said first nozzle being part of said heat source, said second nozzle being part of said liquid supply system. Further according to the apparatus of the invention, said nozzles can be mounted on an arm, said nozzles being movable on said arm and/or said arm being movable relative to said substrate. The heat source can also be a laser beam or another energetic beam instead of a gas nozzle.
In case a heated gas or a heated vapor or a heated mixture of a vapor and a gas is dispensed to locally heat the liquid, the temperature of this heated gas or this heated vapor or this heated mixture is typically in the range from 20 to 200 degrees Celsius. However, the temperature of this heated gas or this heated vapor or this heated mixture is always higher than the temperature of the liquid.